1. Field of the Invention
The present invention relates to a process and a device for the image-differentiated inking of a latent electrostatic image with an inking agent, which is applied to the latent image by the effect of electric fields.
2. Description of the Related Art
In electrographic printing processes, as a rule, a charge pattern is applied in accordance with the image to be produced, then the charge pattern is developed with the help of electrically charged ink particles and transferred to a printing stock.
The development stations comprise arrangements in which charged particles are transferred to the electrographic printing form. The charged particles can be either small charged solid particles (dry toner) or charge particles suspended in a non-polar carrier fluid (fluid toner). Higher resolutions can be attained with fluid toners, because of their substantially smaller particle size.
For dry toners, development stations with magnetic brushes or cascade development are known. For fluid toners, bath development and toner application using rollers or sprayers are known.
In all of these processes, the charged ink (toner) is applied to the entire printing form surface in an undifferentiated manner. Differentiation between image locations and non-image locations is then carried out by means of electrostatic forces. A fundamental problem is keeping the last toner residues away from the non-image locations.
With fluid toners, an additional problem arises in that the non-polar carrier fluid moistens the entire printing form surface and must be removed from at least the non-image locations by expensive processes. From U.S. Pat. No. 4,268,597, a process and a device are known for developing an electrostatic image on the surface of an object (e.g., a roller or printing stock) and the object is moved into the vicinity of the surface of a developer fluid, without touching said surface. However, the strength of the electric field and the distance between the surface of the object and the fluid surface in the area of the development zone are such that, with or without an external electric field, the electric field of the latent image attracts the surface of the developer fluid in the direction of the image portions on the surface of the object. As a result, the fluid surface moves onto the electrically attractive areas of the surface of the object, and the imaging material contained in the fluid is deposited on the latent image. The fluid surface is formed, for example, on a roller (e.g., a screen roller) rotating in a container filled with the developer fluid, and the developer fluid is then removed by a blade to a desired thickness. Between a cylinder that carries the image to be developed and the screen roller there is a small air gap, through which the developer fluid passes to the image areas (i.e., the areas carrying an electrostatic image). The cylinder has a photoconductive coating, for example, which is initially charged by corona discharge over its entire surface. The circumferential surface of the cylinder is then partially neutralized by irradiation with light. At the non-neutralized locations, the circumferential surface attracts the developer fluid, and a meniscus is formed. Instead of imaging the cylinder surface, it is also possible to image a printing web that runs over the cylinder and has on its surface a photoconductive layer on which a latent electrostatic image can be created. The imaging process can be assisted by the application of an electric potential difference between the ink application roller, arranged at a distance from the cylinder, and the cylinder.
Using this process, it is not possible to precisely control the passage of the toner onto the imaged surface, and only electrostatically active fluid toners can be used as the inking agent.
Since 1912, an ink-jet principle has been known, which is described in greater detail, for example, in the article "Continuous Gray-Scale Printing with the Electrohydrodynamic Ink-Jet Principle" by D. H. Choi and F. C. Lee in the IBM Research Report RJ 8913 (80000) of Aug. 10, 1992.
In this process, a fluid (ink) meniscus is produced in a thin electrically conductive tube and held there by capillary forces such that the fluid does not leave the tube. If an electric field of a certain size is applied between this tube and a plate located at a suitable distance from the tube, a very thin liquid jet forms, due to electrostatic forces. The liquid jet stops once the electric field is switched off. By actively switching the electric field between individual tubes and the plate in accordance with the image, it is possible to produce an ink-jet image. This principle is used in ink-jet heads, in which each of many tubes that together constitute the print head can be individually addressed electronically. The tubes typically have diameters between 200 and 500 .mu.m, which attain an ink jet diameter of 15 .mu.m, for example.